Bolt Torque Calculation
Calculate tightening torque from bolt diameter, clamp force, and nut factor — or reverse-calculate clamp force from torque
Calculation Results
Nut Factor K Reference Table
| Surface / Lubrication | K Range | Typical Value |
|---|---|---|
| Dry steel on steel (new bolts) | 0.18 - 0.22 | 0.20 |
| Light oil lubrication | 0.13 - 0.17 | 0.15 |
| MoS₂ (moly) grease | 0.10 - 0.15 | 0.13 |
| PTFE (Teflon) coating | 0.08 - 0.14 | 0.12 |
| Hot-dip galvanized | 0.16 - 0.22 | 0.18 |
| Phosphate coating | 0.12 - 0.18 | 0.16 |
| Cadmium plating | 0.12 - 0.16 | 0.14 |
| Stainless on stainless (dry) | 0.20 - 0.30 | 0.25 |
Common Bolt Grades Reference (ISO 898-1)
| Grade | Size | Proof Load (kN) | Recommended Clamp Force (kN) |
|---|---|---|---|
| 4.8 | M6 | 8.0 | 5.6 |
| 4.8 | M8 | 14.6 | 10.2 |
| 4.8 | M10 | 23.1 | 16.2 |
| 8.8 | M8 | 23.2 | 16.2 |
| 8.8 | M10 | 36.7 | 25.7 |
| 8.8 | M12 | 53.3 | 37.3 |
| 8.8 | M16 | 98.9 | 69.2 |
| 10.9 | M10 | 51.6 | 36.1 |
| 10.9 | M12 | 74.9 | 52.4 |
| 10.9 | M16 | 139 | 97.3 |
| 12.9 | M10 | 61.8 | 43.3 |
| 12.9 | M12 | 89.8 | 62.9 |
Note: Recommended clamp force is set at 70% of proof load. Always consult applicable engineering standards for critical connections.
Bolt Torque Calculation Basics
What is bolt torque?
Bolt torque is the rotational force applied when tightening a bolt, measured in N·m (newton-meters) or lbf·ft (pound-feet). Proper torque ensures the bolt generates the right clamp force — not too loose (risk of loosening) and not too tight (risk of yielding or breaking).
The Core Formula
T = K × D × F
- T = Tightening torque (N·m or lbf·ft)
- K = Nut factor (dimensionless, reflects friction conditions)
- D = Bolt nominal diameter (converted to meters or feet in the formula)
- F = Clamp force / preload (N or lbf)
Metric example: M10 grade 8.8 bolt, clamp force 25 kN, K = 0.20
T = 0.20 × 0.010 m × 25000 N = 50 N·m
Why the nut factor K matters
The nut factor K is the single most critical variable in torque calculations. It's not a pure friction coefficient — it's an empirical value that combines thread friction, bearing surface friction, and thread geometry effects. The same torque can produce clamp forces that differ by 2× or more depending on K. For critical joints, always determine K experimentally.
Frequently Asked Questions
What is the bolt torque calculation formula?
The bolt torque formula is T = K × D × F, where T is torque (N·m), K is the nut factor, D is the bolt nominal diameter (in meters), and F is the clamp force (in N). The nut factor K typically ranges from 0.15 to 0.20 depending on friction conditions.
What is the nut factor K?
The nut factor K is an empirical coefficient that accounts for thread friction, bearing surface friction, and thread geometry in the torque-clamp force relationship. Common values: dry steel on steel ~0.20, lightly oiled ~0.15, galvanized ~0.18, PTFE coated ~0.12.
How do I choose the right clamp force?
Bolt clamp force is typically set at 70-75% of the bolt's proof load. For example, an M10 grade 8.8 bolt has a proof load of about 36.7 kN, so the recommended clamp force is about 25-27 kN. For critical connections, consult standards like VDI 2230 or ISO 898-1.
How do I convert between metric and imperial torque units?
1 N·m = 0.737562 lbf·ft, and 1 lbf·ft = 1.355818 N·m. For diameter: 1 mm = 0.0393701 in, 1 in = 25.4 mm. This calculator supports one-click switching between metric and imperial units.
Why does my actual torque differ from the calculated value?
In practice, K varies with surface condition, lubrication, tightening speed, and thread quality. The torque wrench's own accuracy also plays a role. For critical joints, consider using the torque-angle method or directly measuring bolt elongation to control preload more precisely.